Cylindrical magnetic memory element having plural concentric magnetic layers separated by a nonmagnetic barrier layer

ABSTRACT

A cylindrical magnetic film memory element. Plural concentric film layers are deposited on a cylindrical substrate. The substrate is either a solid conductive cylinder such as a wire or rod, or is a composite cylinder comprising a conductive film deposited on a solid or hollow nonconductive tube support. The magnetic films are either all anisotropic or mixed anisotropic and isotropic. In one embodiment, the anisotropic films have a closed hard axis and the easy or preferred axis of magnetization is parallel with the longitudinal axis of the substrate and the quiescent saturation magnetization within the film layers are disposed in an antiparallel longitudinal manner. In a second embodiment, dual anisotropic films have a preferred easy axis orientation circumferentially directed, thereby giving a closed easy axis.

United States Patent 1 3,576,552

' [5 6] References Cited [72} lnventor Albert W. Vinal 3,440,626 4/1969Penoyer 340/174 Owego, N.Y. 3,516,076 6/1970 Stein 340/174 [21] Appl.No. 693,409 3,518,639 6/1970 Feldtkeller et a1 340/174 [22] Filed Dec.26, 1967 3,518,641 6/1970 DeChanteloup 340/174 [45] Patented Apr. 27,1971 3,488,167 1/1970 Chang et a1 29/1961 [73] Assignee InternationalBusiness Machines OTHER REFERENCES Corporation IBM T h Armonk NY. ecnical Disclosure Bulletin, Coupled Film Memory by Louis, Vol. 7, No. 6,11/64, pgs. 483- 484,

copy in 340 174 Keeper.

Text: Magnetic Thin Films, by Soohoo; Harper & Row

54 CYLINDRICAL MAGNETIC MEMORY ELEMENT il g 2 copy GmuP ScientificHAVING PLURAL CONCENTRIC MAGNETIC LAYERS SEPARATED BY A NONMAGNETICPrimary Examiner-Stanley M. Urynowicz, Jr.

BARRIER LAYER Attorneys-Hanifin and Jancin and John "S. Gasper 10Claims, 5 Drawing Figs. [52] 11.5. Cl 340/174 ABSTRACT: A cylindricalmagnetic film memory element [51] 'f Cl G1 1c 11/14 Plural concentricfilm layers are deposited on a cylindrical [50] Field of Search 340/174;Substrate The Substrate is either a Solid conductive cylinder 307/88such as a wire or rod, or is a composite cylinder comprising aconductive film deposited on a solid or hollow nonconductivetube'support. The magnetic films are either all anisotropic or UNITEDSTATES PATENTS mixed anisotropic and isotropic. In one embodiment, the

3,451,793 6/1969 Matsushita...... 340/174X anisotropic films have aclosed hard axis and the easy or 3,480,929 11/1969 Bergman.... 340/174preferred axis of magnetization is parallel with the longitu- 3,188,6136/ 1965 Fedde 340/ 174 dinal axis of the substrate and the quiescentsaturation mag- 3,193,694 7/1965 Ehresman et a1. 4 307/88 netizationwithin the film layers are disposed in an antiparallel 3,358,273 12/ 1967 Henninger et al. 340/174 longitudinal manner. In a secondembodiment, dual anisotrop- 3,370,979 2/1968 Schmeckenbecker 340/174X icfilms have a preferred easy axis orientation circum- 3,375,09l 3/1968Feldtkeller 340/174X ferentially directed, thereby giving a closed easyaxis.

I PATENREURRRNRR V 3576552 SHEETZDFB V RRISRRR conouc ISOT 0 TIME CONS m+(T)i nanoseconds BARRIER LAYER THICKNESS (Z) I PATENTEMRQ? i'sn sntefinr 5 FIG. 13

RESPONSE IN E M V PEAK 0- FILM THICKNESS INA barrier layer is made ofconductive material and is thick enough to support circumferential eddycurrents generated when a current is applied to the substrate for readand write operations. A solenoid field parallel with the longitudinalaxis of the substrate is generated by the' eddy current in the barrierlayer and operates to effect a phase displacement between themagnetization vectors to produce an output signal in a sense conductorlayer concentrically superimposed adjacent to the outer' magnetic filmlayer. The conductive barrier layer thickness is varied as a means forcontrolling the magnitude of vector phase difference and, hence, theoutput signal level in a bit-sense conductive means. The barrier layermay be nonconductive and the dynamic magnetic vector phase differentialis obtainable if the separate films have magnetic properties such thatthe angular rotation rate of magnetization within the magnetic films isinherently different. The memory element has particular application in aword organized NDRO memory in which the conductive substrate is the wordline and a bit or bit-sense loop is disposed concentrically with andadjacent to the outermost magnetic film area.

In the closed easy axis embodiment, the magnetization vectors arecircumferentially parallel. The barrier layer between the pair ofmagnetic layers is preferably conductive and is relatively thin topermit transverse magnetic flux coupling between the magnetic layers,thereby preventing domain wall creepwithin'the magnetic layers. Thismemory element hasparticular application in word organized DRO memoryconfigurations.

DESCRIPTION OF THE PRIOR ART Data storage. devices have been devisedwhich use discrete magnetic film elements as the storage means. Themagnetic film elements have taken the form of rectangular planar filmswith either single or multiple layers, as well as cylindrical films on acylindrical substrate comprised of one or more concentric magneticlayers. in all cases, the films and layers have the property of beinguniaxially anisotropic and their operation as storage devices depends onthe ability to rotate the direction of magnetic saturation within thefilms when subjected to ex- ..ternally controlled magnetic fields.

ln the past, uniaxial anisotropic film elements have suffered I from alack of magnetic stability, The instability has been found to beattributable to the existence of relatively large demagnetizing fieldswithin the film. in the film configurations mentioned, the demagnetizingfield exhibits a component 4 parallel to and opposite in direction tothe saturation mag- -n etization of the film. Notonly does switchingrequire increased energy toovercome this field component, but themagnetization within the film, particularly in its central region, canbe disordered if the parallel demagnetizing component were to approachthe value of the coercivity of the film which is characteristically lowin such films. In a rectangular planar film, the demagnetizing fieldalso exhibits components orthogonal to the anisotropic axis which leadsto uncontrollable switching in the comers of the film to thereby renderthe film unreliable for use as NDRO storage elements. While the closedloop magnetic film has minimized the effect of the orthogonaldemagnetizing field components, the parallel component still exists.

It has been found that the demagnetizing field intensity is quitenonlinear and is directly proportional to the thickness of the film andinversely proportional to film length along which magnetization isdirected. Consequently, to avoid the undesirable effects of thedemagnetizing fields which caused intioned defects.

stability, the films were kept relatively thin and long. As a practicalmatter, such films were limited to a thickness of 1000 A. or less with alength of 30-50 mils. Such thin films, however, have characteristicallyexhibited lowoutput signals when switched, and are quite sensitive tostray fields, thereby limiting their use in many DRO and NDROapplications. Likewise, applications requiring a high storage densitywould not be attracted to such film elements.

Not only did the demagnetizing field adversely affect the magnetostaticproperties of the film, but also the dynamic properties. Relativelylarge drive current well controlled in amplitude was required togenerate a magnetic field sufficiently strong to read out or change thestate of the film since-the demagnetizing field also had to be overcomein rotating the saturation magnetization within an anisotropic film fromits easy axis to its hard axis of magnetization. Furthermore, the upperlimit of the word drive current required to effect such switching wasvery restrictive,,particularly with respect to NDRO operation. Whiledual concentric layer magnetic film elements have been devised as ameans to improve switching and the output signal, switching level,switching speeds tolerance to variation in word current pulse amplitudeand stray fields and thermal stability continue to be relatively poorfor many applications and the overall magnetic stability has remained aproblem, particularly in NDRO applications.

SUMMARY OF THE INVENTION It is an object of this invention to provide animproved magnetic film memory element which overcomes the above-men- Itis a specific object of this invention to provide a magnetic whichrequires lower switching energy, which has improved magnetodyna'micproperties to attain larger response signals at higher switching speeds,and whose information state is tolerant of wide variation in wordcurrent amplitudes during energization.

It is a further object to provide a magnetic film memory element whichis not thickness and length limited and which is capable of being usedin high density storage arrays.

It is also an object to provide an improved magnetic film memory elementwhich is economical to fabricate and highly reliable for use in avariety of storage applications.

it is a still further object to provide an improved magnetic film memoryelement capable of reliable use in both DRO, NDRO and electricallyalterable read only storage applications.

It is an additional object to provide a bistable magnetic film memoryelement having improved stability over a relatively wide temperaturevariation.

The above, as well as other objects, are attained in accordance withthis invention by a cylindrical memory element comprising a smoothconductive cylindrical substrate, preferably of circular cross section,carrying two or more concentric magnetic film layers separated by abarrier layer. In a first embodiment, at least one of the magnetic filmsis anisotropic and has a preferred direction of saturation magnetizationparallel to the longitudinal axis of the substrate. The other film(s)may be isotropic or anisotropic and in the latter case, the preferredaxis of magnetization is also parallel with the longitudinal axis of thesubstrate. In both cases, the film layers are antiparallel coupled. Thisantiparallel magnetic couple results in a mutual cancellation of thelongitudinal (easy axis) demagnetizing field component within the filmsforming bits of discrete length..All hard axis demagnetizing fieldcomponents are eliminated since each of the concentric magnetic layersform a closed circumferential magnetic loop giving rise to a memoryelement completely magnetostaticallystable. The barrier layer is thickenough to eliminate magnetostatic exchange coupling between the magneticfilm layers and thereby permits magnetization within the separatefilmsto be rotated freely and independently during energization and to assumeantiparallel disposition during the quiescent condition.

Cancellation of all demagnetizing fields permits the use of films havingthicknesses in the range exceeding 1000 'A. up to 50,000 A. therebypermitting the production of relatively large readout or sense signalgeneration. The information statesof the element correspond to thequiescent disposition of the magnetization within the top film.Magnetization within the lower film(s) always being antiparallel withrespect to the top. There is no magnetostatic restriction limiting thelength of the films forming the bits and a practical increase in storagedensity is thereby obtainable. Because all hard axis demagnetizing fieldcomponents have been eliminated, relatively small drive currents can beused to effect coherent magnetization rotation and such rotation can beachieved at higher speeds.

In a first embodiment of this invention, the memory element comprisestwo concentric anisotropic magnetic layers with their easy axes ofmagnetization directed mutually parallel with the longitudinal axis of aconductive cylindrical substrate. The intrinsic anisotropy fieldstrength of the concentric layers is preferably substantially equal andthe barrier layer is conductive and of a thickness great enough toprovide a circumferential shorted turn for eddy currents to flow asgenerated by rotation of the magnetization within the underlying films.As a result of the circumferential currents in the barrier layer, asolenoid field is generated which is parallel with the longitudinal axisof the substrate and which modifies the angular rotation rate ofmagnetization within the underlying magnetic film to produce a dynamicphase displacement between the rotation angles of the magnetizationvectors of both layers. Since theamplitude of the output signal isdirectly proportional to the phase differential in the angular rotationrate of the antiparallel magnetization vectors, the conductive barrierlayer provides a means for obtaining an increased signal output. In amodification of this embodiment, the concentric magnetic films havedifferent magnetic properties so that their intrinsic anisotropyfieldstrengths are not equal. In this condition, the rotation rate ofthe magnetization vectors is inherently different to produce a vectorphase differential. The barrier layer may be conductive or nonconductiveand, in either case, is thick enough to eliminate magnetic exchangecoupling between the layers.

An additional modification of this embodiment consists of threeanisotropic magnetic film layers. The first and second magnetic filmlayers have a combined magnetic thickness substantially equal to themagnetic thickness of the third or outermost magnetic film. Each filmhas its easy magnetic axis parallel with the longitudinal axis and areseparated from one another by a conductive barrier layer. The magnitudeof the intrinsic anisotropy fields within the magnetic films need not beidentical. The magnetization vectors within the first and second filmlayers are oriented parallel with respect to one another andantiparallel with respect to magnetization within the outermost filmlayer.

In a second embodiment of this invention, the anisotropic concentriclayer is combined with one or more isotropic layers mutually separatedby a barrier layer. The easy axis of the anisotropic layeris once againparallel with the longitudinal axis of the substrate. Because of themagnetostatic interaction between the anisotropic and isotropic layers,an induced antiparallel magnetic couple is formed to mutually cancel thelongitudinal demagnetizing field component. The barrier layer may beeither conductive or nonconductive and in either event is thick enoughto eliminate magnetic exchange coupling between the anisotropic andisotropic films. Because of the difference in the intrinsic anisotropicfield characteristic of the films, the angular rotation rate in thefilms will inherently produce a phase differential. Where the barrierlayer is conductive, the solenoid field effect is useful for obtaining afurther change in phase differential with consequent amplification ofsignal output level.

In a modification of the second embodiment, the cylindrical memoryelement comprises an anisotropic layer sandwiched between inner andouter concentric isotropic layers with barrier layers separating thevarious layers. The'thickne'ss of the combined isotropic layers is atleast equal to thethickness of the anisotropic layer.

In a further embodiment, the memory element comprises concentricmagnetic layers each having a closed easy axis, i.e., the preferredmagnetic axis, of orientation are circumferential relative to thecylindrical substrate. The magnetization vectors in the magnetic layersare parallel. A concentricbarrier layer separates the magnetic layersand is preferably conductive. The thickness of the barrier layer is suchthat transverse magnetic flux coupling between layers is permittedthereby providing a magnetic storage element which is not sensitive,i.e., domain wall creep is prevented when the magnetic element issubjected to alternating stray field noise signal which results as partof a word organized magnetic memory.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

In the drawings:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. la is a schematic diagram of aplanar thin magnetic I film illustrating the action of the demagnetizingfield H FIG. 1b is a graph illustrating the theoretical hysteresisproperties of the anisotropic film in FIG. la where demagnetizing fieldeffects are neglected;

FIG. 2 illustrates a planar film improvement of FIG. la;

FIG. 3 shows a single layer cylindrical magnetic film memory element;

FIG. 4 is a graph illustrating the relationship of demagnetizing fieldand bit length for a single layer cylindrical magnetic film of variousthicknesses;

FIG. 5 is a preferred embodiment of the invention wherein twocylindrical anisotropic magnetic films are separated by a conductivebarrier layer;

FIG. 6 is a cross-sectional view of the memory element of FIG. 5;

FIG. 7 illustrates the manner in which unidirectional word andbidirectional bit fields are applied to a memory element in a memorymatrix;

FIG. 8 is a graph showing the rotation magnetization within the magneticfilm circumscribed by the copper barrier layer. The rotation timeconstant is plotted as a function of barrier film and magnetic filmthickness;

FIGS. 9 and 10 illustrate a second embodiment of a cylindrical magneticfilm memory in which one of the magnetic film layers is anisotropic andthe other is isotropic;

FIG. 11 is another embodiment of the invention in which the isotropicfilm is split into two layers on either side of the DESCRIPTION OF THEPREFERRED EMBODIMENTS A problem with magnetic films of discrete lengthis that a demagnetizing field H which is proportional to the thicknessof the film and inversely proportional to the length of the film isdeveloped internal to the film being most intense at the'ends of thefilm. Films suitable for use in memories are characterized by having arelatively low coercivity 11 and, consequently, H may approach or exceedH particularly at the time constant of the extremities ofthe film, andrender the film unsuitable for use "as a reliable storage element. InFIG. la, there is shovm a direction or axis 12 of saturationmagnetization M,. The film length L and thickness d are finite, forexample, 30 mils and 1000 A., respectively. When the element issaturated in the direction shown, the element 10 may be considered tocontain magnetic charge dipoles which lead to demagnetizing fields Hexperienced across the length of the element. The demagnetizing fieldmeasured at the centerline 14 of the element is least intense and isdefined as H If the effect of the demagnetizing field is ignored, theexternal field required to rotate the magnetization vector M, 90' fromthe easy axis equals the intrinsic anisotropic field H However, theactual external magnetizing field H which must be produced by a wordcurrent I to coherently switch a substantial portion of the element 10because of the existence of the demagnetizing field is expressed asfollows:

H =AH +H eq. l where A is a constant defining position between the filmcenter and boundary edges. In a typical film, the term AH may be severaltimes larger than H FIG. 1b illustrates the square hysteresis loopcharacteristic of an ideally magnetized film 10 which is uniformlymagnetized across its area with no components of magnetization M, in anydirectionother than that indicated by the easy axis 12. However, becauseof the magnetic pole charge distribution along the top and bottomsurfaces of film l0, demagnetizing fields orthogonal to easy axis 12, asindicated by the arrows 16, occur in the comer areas of film 10. Theseorthogonal fields cause irreversible switching of the comers of theanisotropic film when the magnetization vector M is rotated from itseasy axis toward its hard axis orthogonal to the easy axis 12.Nondestructive readout of the film is accomplished by rotating themagnetization vector M, by means of an external field H appliedorthogonal to the easy magnetic axis to produce a flux change whichproduces a voltage in a sense winding proximate the film. When the fieldH is released, the magnetization retums to its original direction.However, the orthogonal demagnetizing fields, represented by arrows 16,will cause the magnetization in the corners of the film 10 to passthrough the hard magnetic axis, thereby changing the magnetization statein portions of the film and decreasing the reliability of thenondestructive readout of the film. The orthogonal demagnetizing fieldsalso exist where multiple planar films are superimposed, such asillustratedin FIG. 2 where films and 21 are both anisotropic. The arrows22 and 24 indicate the direction of saturation magnetization vectorswithin their respective films. In this configuration, the netdemagnetizing field 11,, is substantially eliminated in each of thefilms except at the extremities thereof. Orthogonal demagnetizing fieldsstill exist at the comers of the film 20 and 21 as shown by the arrows26 and thereby produce the irreversible switching previously mentioned.

FIG. 3 shows a cylindrical memory element 30 having a single layermagnetic film 31 of finite length disposed on the surface of acylindrical conductive filament 32. The film has uniaxial anisotropicproperties with the easy axis parallel with the longitudinal axis of theconductive cylinder 32 as indicated by arrow 33. The saturationmagnetization M within the film is oriented along the easy axis. Alongitudinal demagnetizing field H exists with its direction-as showninternal and external to the magnetic film layer. This demagnetizingfield occurs due to the discontinuity in magnetization which occurs atthe ends 34 of the film layer 31 forming its length. The intensity ofthis demagneti'zing field varies as a function of the position withinthe film 31 being most intense for all positions close to the film edges34. The film area of the type shown in FIG. 3 is unusable as a binarystorage cell if the intensity of H measured internal to the film 31 at aposition equal in distance from either edge 34 is equal to the coerciveforce H of the magnetic material comprising the film.

FIG. 4 shows the variation of intensity of the demagnetizing field H atthe center of the cylindrical film 31 as a function of the magnetic filmlength and the magnetic film thickness for the values given. In FIG. 4,the saturation M of the film 31 is assumed to be l0,000 gauss. Forpractical bit lengths, a single layer magnetic film 31 with axialorientation of magnetization cannot have a thickness much in excess of1000 A. The amplitude of the response signal obtained from such a filmis at most a few millivoits. One severe operative restriction is thatthe single layer memory element 30 fails to provide stable NDROoperation, even with ideal film properties.

In FIG. 5, the memory element 50 of the invention has a. conductivecylindrical substrate 51 on which are deposited a pair of concentricmagnetic film layers 52 & 53 separated by a continuous nonmagneticbarrier layer 54. In the embodiment of FIG. 5, the magnetic layers 52 &53 are both uniaxially anisotropic with their preferred or easy axisdirected parallel with the longitudinal axis of the substrate 51, asshown by arrow 55. Each of the layers 52 & 53 is a magnetic materialhaving a coercivity to anisotropy field ratio (H /H within the range of0.5 to 1.3 with a skew angle ratio less than 3 and having amagnetostriction less than 2 l06 and preferably substantially zeromagnetostriction. In the preferred embodiment, both layers 52 & 53 areof equal thickness and have equal intrinsic anisotropy fields. In such astructure, the circular cross section of the films 52 & 53 provides aclosed loop or divergenceless magnetic contour which eliminates alldemagnetizing fields orthogonal to the easy axis so that the netdemagnetizing effect, due to orthogonal demagnetizing fields, is zero.Consequently A in equation 1) is zero.

While a solid conductive filament, such as copper wire or rod, is thepreferred form for substrate 51, a composite substrate having adielectric core with a conductive surface may also be used. In addition,dielectric film layers (not shown) may be provided between the variouslayers and the substrate.

It is a further characteristic of the memory element 50, that themagnetic layers 52 & 53 are antiparallel magnetostatically coupled; thatis, the saturation magnetization vector M, of layer 52 is parallel andopposite in direction to the saturation magnetization vector M of layer53-as shown in FIG. 6.

As a consequence of the easy axis antiparallel magnetic couple, thelongitudinal self-demagnetizing fields H generated within each of thefilm layers 52 & 53 become oriented so as to mutually cancel the effectsof each other. This condition may be obtained independently of filmthickness provided the following relationship is observed:

Where D, is the thickness of film layer 52 and D, is the thickness offilm layer 53.

To obtain adequate mutual demagnetization field cancellation, thethickness D of the barrier layer 54 should be less I than 5Xl0'XL, whereL is the length in mils of the magnetic cylindrical bit formed by themagnetic layers 52 and 53. The mutual cancellation effect is notobtainable where the layers 52 & 53 are physically in contact. Inaccordance with this invention, the barrier layer 54in excess ofapproximately 200 A. is required.

One of the functions of the barrier layer 54 is to provide a means forsubstantially eliminating magnetic exchange coupling between themagnetic layers 52 & 53. An understood in connection with thisinvention, the term magnetic exchange coupling is meant a condition thatexists as a result of a strong interaction between adjacentatomicmagnetic moments within the magnetic material. The origin of thisexchange coupling is believed to be the combined effect of spin-orbitinteraction and exchange of coulomb interaction between neighboringorbits.

One of the efi'ects of the exchange coupling is that a torque onneighboring dipole moments exists which opposes independent rotation ofthe magnetization vectors M, and M, of the magnetic layers 52 & 53, ifno barrier layer is provided. By providing the nonmagnetic barrier layer54, magnetic exchange coupling is prevented thereby freeing themagnetization vectors M and M for free independent rotation within theirrespective films. It has been found experimentally that a separationdistance between film layers of 200 A. or greater is sufficient toeliminate the exchange mechanisms between the film layers.

An additional function of the barrier layer 54 in connection with thisinvention is to provide a dynamic solenoid field which acts to modifythe angular rotation rate of the magnetization vector M, within themagnetic film 52. This function requires that the barrier layer 54 notonly be nonmagnetic and a conductor, but that it have a thickness greatenough to permit circumferential eddy currents to be generated due tothe rotation of the magnetization vectors.

The principle of the solenoid field effect as a means for controllingthe rotation vector M,, within the magnetic film 52 is better understoodby reference to FIG. 7. In FIG. '7 the memory element 50 is one of aplurality of such elements forming a word organized magnetic memoryarray. In the array, plural discrete magnetic elements are formed atspaced locations along the substrate 51 and plural such conductorshaving an equivalent number of memory elements 50 are at ranged in aplanar array. An example of such an array is given in copendingapplication, Ser. No. 635,072, now US. Pat. No. 3,487,372, issued onDec. 30, 1969. A bit-sense loop 56 comprises conductive layers 57 and 58preferably deposited on an insulating sheet 59. The terminals of saidbit-sense loop is connected to appropriate sense amplifier and bit drivecircuits (not shown). The substrate 51 is preferablya word conductorconnected to a suitable energizing drive circuit which produces an axialcurrent pulse l The unidirectional word current 1 flowing axially alongthe word line 51, for example, when the memory is operated NDRO,produces a circumferential magnetizing field H which is orthogonal tothe easy directions of magnetization vectors M, and M Thecircumferential magnetic field H causes the magnetization vectors M, andM,, to be rotated in opposite angular directions toward their respectivehard magnetic axis. The amplitude of the response signal in thebit-sense loop 56 is proportional to the time rate of change of the netmagnetization encompassed by the loop, but since the magnetizationvectors M, and 'M, within the film layers 52 8t 53 are orientedantiparallel with each other, and due to the symmetry of the bit-senseloop, 56,

the net magnetization encompassed by the loop is numerically zero wherethe magnetic layers 52 & 53 have identical magnetic properties. In orderfor a net sense signal to be generated in the conductive bit-sense loop56, a difference in rotation rate must exist between the magnetizationvectors M and M as they rotate from the easy axis 55 toward the hardaxis of their respective layers. As the magnetization vectors rotate byforce of the orthogonal word field H a circumferential eddy current isinduced in the barrier layer 54 which develops a solenoid field parallelto the longitudinal axis of the substrate 51 and parallel with the easyaxis of the inner magnetic layer 52. The solenoid field componentoperates to oppose the rotation of the magnetization vector M,,.of theinner film 52 while its effect on the rotation rate of magnetization Mwithin the outer layer 53 is essentially negligible. The rotationalvelocity of the magnetization M in the inner magnetic layer 52 isthereby retarded relative to the rotational velocity of magnetization M:in the outer film 53. The bit-sense loop 56, accordingly, experiences adifferential in the rotation rates and a net sense signal is generatedat the terminals of the bit-sense loop 56. A time constant T whichgoverns the rate of magnetization rotation is plotted in FIG. 8 as afunction of the thickness of conductive barrier layer 54 for variousthicknesses of magnetic layer 52. Mathematically, it has been determinedthat the maximum damping time constant is reached when the angulardisplacement of magnetization in the lower layer 52 reachesapproximately 54 relative to'the easy axis. For the purposes ofcomputation, the saturation magnetization of the magnetic layer 52 wasalso established as 10,000 gauss and the resistivity of the barrierlayer .was 1.73Xl ohms -cm. which is the resistivity for copper. As

seen in the graph of FIG. 8, foTzTcopper barrier layer of 6000 6000 A.thick where the magnetic layers were each made of NiFe 8000 A. thick.The damping time constant is 70 nanoseconds when the lower film 52rotates 54 from its easy axis. Some damping of the lower film 52 isproduced by the second magnetic film layer 53. However, this is at leastone order of magnitude less effective than the damping achieved by thecopper barrier layer 54 of equivalent thickness.

In order to write anew binary state into element 50 or destructivelyread out the given state, bit current pulses 1,, are applied tobit-sense loop 56 as the magnetization vectors reach the hard axis. Thebit field H produced by 1,, will then cause magnetization within theanisotropic film 53 to continue rotating beyond the hard axis leading toan antiparallel disposition of magnetization vectors displaced 180 fromtheir starting positions. By this means, the direction of the bitcurrent determines the information state of the element. In order toachieve this state reversal, the bit field H, is caused to be terminatedafter the termination of the word field H Dynamically, the word field Hwhich must be applied to rotate the magnetization vector M, in order tostore bits in the memory element 50 is determined by equation (1).However, the dual magnetic film structure of FIG. 7 reduces the hardaxis demagnetizing field H to zero, so that the word field H mustovercome only the intrinsic anisotropic field H, of the film 52 torotate the magnetization vectors from the easy axis to the hard axis.Consequently, the write current 1 required to produce the field H ismuch less than required for prior art' devices in which the effect ofthe hard axis demagnetizing field must also be overcome by the fieldproduced by the word current. The elimination of H in both hard and easydirections also permits the use of thicker magnetic films which confinemore flux so that larger sense signals are. produced upon switching of afilm.

FIG. 13 shows the peak amplitude response signals obtained from aspecific example of a bistable magnetic device of the type shown in FIG.5 which embodies the principles of the present invention. Curve 110shows the signal response on the bit-sense line 56 (see FIG. 7) for aone data bit while curve 111 is the signal response for a zero data bit,when the rod 51 was energized with a word current pulse (l )=720 ma.with a rise time of approximately 9 nanoseconds. The one and zero curvesare shown superposed for convenience in illustration. Curve 112represents background or noise signals appearing on the bit-sense line.In the example illustrated, the magnetic device was operated NDRO wherethe repetition rate was 10 megacycles. In the specific exampleillustrated in FIG. 13, a bistable magnetic storage device was madeusing a 20 mil. OD polished beryllium copper rod as a conductivesubstrate. Deposited on the rod was a film having dual magnetic layers-20 NiFe each having a thickness of 12.5 KA. Both mag netic layers had aHe and H magnitude of approximately 4.0

' oersted while both had an easy axis orientation parallel with thelongitudinal axis of the rod. The skew and dispersion angles for bothlayers was less than 1 and the magnetostriction was estimated to be inthe order of less than 2X10". The magnetic layers were separated by afine grain copper layer having a thickness of 12 KA. Storage bits on thesubstrate were formed in lengths of 60 mils by etching.

FIG. 14 is a plot for additional examples of cylindrical magneticstorage devices showing the relationships of peak response signal forvarious word currents as a function of thickness of the identicalmagnetic layers where other structural parameters are essentially thesame as in the previous example and where word current (1 has a risetime of approximately 10 nanoseconds.

It should be recognized that the response signal amplitude and wordcurrent are also directly proportional to the substrate diameter. A 5mil rod will produce one-fourth the signal and require one-fourth theword current to operate.

While in the above examples a specific magnetic material having aspecific ratio is specified, other magnetic materials can be used topractice the present invention. In addition,

where NiFe materials within the group known as Permalloy are used,theratios of NiFe can vary within the. interval 70 percentfiNifFe83percent with the ratio of 80 percent Ni 20 percent Fe being preferred.When cobalt is utilized to increase the intrinsic anisotropy field ofone or more of the film layers, a value of percent by weight or less ispreferred. A composition of 78 percent Ni 19 percent Fe 3 percent Co hasbeen found to be satisfactory.

. In another form of the first embodiment, the magnetic layers 52 & 53of the memory element 50 are also both anisotropic with their easy axesparallel with the longitudinal axis of the substrates 51, and areantiparallel coupled; however, the nonmagnetic barrier layer 54 used toseparate the films is nonconductive. In order to produce a signal outputwhen magnetic switching takes place, the inner and outer anisotropicmagnetic layers 52 8t 53 are made from materials having substantiallydifferent intrinsic anisotrophy fields. For example, referring to FIG.5, the inner magnetic layer 52 would be formed of a ferromagneticmaterial such as NiFe and Co having a coercive force and anisotrophyfield of approximately 7.0 oersteds. The outer magnetic layer 53 wouldbe made of an alloy comprising NiFe having a coercive force andanisotrophy field of approximately 4 oersteds.

In this embodiment, the barrier layer 54 could be a dielectric film suchas silicon monoxide. The thickness of the dielectric film would be greatenough to eliminate the magnetic exchange coupling between the magneticlayers, but need not be asthick as a conductive barrier layer since eddycurrent generation is not necessary to produce the rotation ratedifferential needed to generate an output signal in the bit-sense loop56. In this particular embodiment, the rotation rate differential isachieved due to the inherently different magnetic properties of theanisotropic layers 52 & 53 as their respective magnetization vectors arerotated in the antiparallel mannerfrom the easy axis toward theirrespective hard magnetic axes. However, the efficiency of the responseexcitation properties will not be as good as the first embodiment whichutilizes identical films and a conductive barrier.

In FIG. 9, there is illustrated a second embodiment of the memoryelement 60 of this invention in which the concentric magnetic layers 62& 63 on the conductive substrate 61 are isotropic and anisotropic,respectively. The magnetic layers 62 and 63 are separated by aconductive barrier layer 64. The preferred or easy axis 65 of saturationmagnetization of the anisotropic film 63 is parallel to the axis ofsubstrate 61. A longitudinal magnetization vector M is induced in theisotropic layer 62 in a direction opposite to the magnetization vectorM,,, of anisotropic layer 63. Thus, antiparallel magnetic coupling isobtained which cancels the longitudinal demagnetizing field componentsin both films.

As is the case of the embodiment where both films are anisotropic,barrier layer 64 is made thick enough to permit circumferential eddycurrents to flow therein. These eddy currents are generated in layer 64by the rotation of the magnetization vectors M, and M within the filmlayers 62 & 63 when the memory element is subjected to a circumferentialword magnetizing field produced by an axial word current in theconductive substrate 61. When the word field is applied, themagnetization vector M of film 62 will start to rotate ahead of themagnetization vector M,,,, of film 63. However, as M, begins to rotate,it generates eddy currents in conductive barrier layer 64 which thenacts as a solenoid to produce a solenoidal field which retards therotation of M Consequently, M will lag behind M and the resultant phasedifference produces a change in flux which will induce a signal voltagein a bit-sense loop (e.g. loop 56 of FIG. 7) associated with the memoryelement 60.

'In the preferred embodiment of FIG. 9, the magnetic film layers 62 8t63 may be from 1000 A. to 50,000 A. thick and 10 mils or greater inlength. The thickness of conductive barrier layer 64 is from 200 A. toapproximately SXIO L, where L is the discrete length of the magneticlayers 62 8t 63 in mils.

ill

In this second embodiment of the invention, both film layers 62& 63 arepreferably of the same magnetic thickness. However, by using themagnetic antiparallel coupling and cylindrical shape of this invention,where one of the films which possesses isotropic properties, the filmsmay be of unequal thickness and the saturation magnetization may beunequal so long as the following equation is satisfied:

where:

M is the saturation magnetization of the anisotropic film layer,

M is the saturation magnetization of the isotropic film layer, I

t and t, are the thickness of the anisotropic and isotropic film layers,respectively,

6, rest angle between longitudinal axis of the cylindrical rod andmagnetization vector of the isotropic film layer.

in another form of this embodiment of the invention, the barrier layermay be as thin as 200 A. which is that value sufficient to eliminateexchange coupling between the magnetic films. However, with such a thinbarrier layer, sufficiently large eddy currents are not produced thereinto cause the vector M, to lag behind vector M when the memory element issubjected to the word field H The resultant phase displacement producesa change in flux which may be detected by a suitable sense conductor. Inthis form, thebarrier layer need not be conductive.

FIG. 10 shows a memory element 70 in which the inner magnetic film layer72 is anisotropic having a longitudinal easy axis 75 and the exteriorfilm 73 is isotropic, which is the reverse arrangement shown in memoryelement 60 of FIG. 9. In addition, the substrate 71 comprises a soliddielectric core 76 on which is deposited a conductive film 77. Theoperation of the memory element in FIG. 10 is similar to that in FIG. 9in that a solenoid field is generated by circumferential eddy currentswithin the conductive barrier layer 74 to retard the rotation of themagnetization vector M of the film 72. Since the magnetization vector Mof the film of isotropic film 73 tends to lead the vector offilm 72 whencurrent b is applied to conductor 77, the delay produced by thelongitudinal solenoid field from barrier layer 76 tends to produce aneven greater phase differential between M a and M and the sense signal Ein a bit-sense loop would be' according ly modified.

FIG. 11 shows a memory element 80 which illustrates a further embodimentof the invention using three concentric layers 82, 83, & 84 deposited ona hollow cylindrical conductive substrate 81. The inner and outer layers82 and 84, respectively, are both isotropic while the sandwiched layer83 is anisotropic with its easy axis 85 parallel to its longitudinalaxis of the substrate 61. Conductive barrier layers 86 & 87 separate theanisotropic layer 83 from the isotropic layers. An induced antiparallelmagnetic coupling is produced between the magnetic layers such that themagnetization vectors of the isotropic layers 82 & 84 are mutuallyparallel with the longitudinal axis of the conductor 8B. In thisembodiment, the combined thickness of the isotropic layers 82 & 84 is atleast equal to, but can be greater than, the thickness of theanisotropic layer 83. This is to assure that the induced angularvelocity of the magnetization vectors within the isotropic film layers82 & 84 will rotate at a faster angular rate than that within theanisotropic film layer 83. The angular rotation rate in the isotropicfilm layers 82 & 84, when an energizing word current is applied toconductor M, can reach up to four times that within the anisotropiclayer 83 as a result of the division of the isotropic layer by theanisotropic layer when the thickness ratio specified above ismaintained. To obtain a measure of control over the signal outputconductive barrier layers 86 8t 87 operate to modify the rotation rateswithin the inner layers 82 8t 83.

FIG. 12 shows a memory element 90 which illustrates still anotherembodiment of the invention in which two uniaxial magnetic film layers92 and 93 act in combination as a magnetostatic keeper for uniaxialmagnetic film layer 94. All easy magnetic axes of magnetization areparallel with the longitudinal axis of the substrate.'Conductive barrierlayers 95 and 96 decouple the exchange forces between-film layers 92 and93 and 94 respectively. Magnetization within magnetic films 92 and 93are mutually parallel and are oriented antiparallel with magnetizationwithin magnetic layer 94. The magnetostatic requirements of thisembodiment are satisfied in accordance with the following equation:

Ms S!,, +Ms St =Ms St eq. (4) Where Ms is the magnetization vector oflayer 92 Ms is the magnetization vector of layer 93 Ms is themagnetization vector of layer 94 S1 S1 and St are the thickness of thelayers 92, 93, & 94,

respectively.

The operation of this of the first embodiment of the invention as shownin FIG. 7.

' The principal difference lies in control of the wall motion propertieswithin layers 92 and 93 which are effective when changing states duringwriting. The preferred design for this embodiment is the magneticthickness of films 92 and 93 are to be equal and that their combinedthickness satisfy eq. (4). The conductive barrier layers 95 and 96 neednot be equal in thickness and should satisfy the exchange decouplingcriteria set forth in previous paragraphs of this description.

In FIG. 15, the memory element 100 has a conductive cylindricalsubstrate 101 on which are deposited a pair of concentric magneticlayers 102 & 103 separated by a nonmagnetic barrier layer 104. In thisembodiment, the magnetic layers 102 & 103 are uniaxially anisotropicwith their preferred or easy axis 105 directed circumferentiallyrelative to the substrate 101. The memory element is therebycharacterized as having a closed easy axis for both magnetic layers 102& 103. Thebarrier layer 104 is preferably copper. In this embodiment,magnetization vectors Ms and Ms are parallel for use in storing data andthe barrier layer 104 is relatively thin to permit transverse wallcoupling whereby the magnetization vectors device is quite similar tothe operation rotate together upon application of a word pulse. In thisembodiment, the substrate serves as a bit-sense line while the wordlines, (not shown) which would take the same form as the bit-senseconductive loop 56 of FIG. 7, are orthogonal to the substrate 101. Inthis embodiment, the magnetic layers are preferably of equal magneticthickness and have substantially identical magnetic properties and areformed from NiFe alloys or the like. While two magnetic layers are shownin FIG. 14, additional concentric layers in even numbers of magneticfilms may be applied. The magnetic films preferably have a coerciveforce H which is equal to or less than the anisotropy field H of thematerial deposited. The barrier film is preferably fine grain copperdeposited with a thickness in the range of from 100 to I000 A. with thepreferred thickness being about 600 A. The total thickness of themagnetic layers comprising magnetic storage device 100, is preferably inthe order of 6000 A. If it is desired to have a magnetic film of greaterthickness to get increased signal amplitudes, additional layers in evenmultiples are provided. With the above construction, magnetic elements100 exhibit good disturb properties where alternating noise signalcurrents appear on the substrate when multiple data magnetic bits appearon a common line in a word organized DRO memory. While the specificreason for this is not completely understood, it is believed that thethin barrier layer permits transverse wall coupling between magneticlayers 102 & 103 thereby preventing Bloch wall creeping when disturbsignal fields appear.

While various techniques for depositing the various film layers areknown, the preferred techniques for making the cylindrical elementsemploys an electroless bath comprised of an aqueous solution of Iron andnickel salts in the presence of reducing agents such as sodiumhypophosphite and a complexing agent such as sodium potassium tartrate.The bath preferably includes added amounts of NH,C1 and NH OH with a pHadjusted to be in the range of 7-l3 preferably at about 10.5 while thetemperature of the bath is maintained within the range of l5-45 C. with32-3 8 preferred. For depositing NiFeCo. layers, a cobalt salt is addedto the solution. Further details of suitable electroless baths and amethod of preparation can be obtained by preference to copendingapplication Ser. No. 678,890 of H. N. Rader, A. W. Vinal, and L. R.Yetter, filed on Oct. 30, 1967, and assigned to the common assignee.

While various techniques may be used to produce isotropiccharacteristics to the various layers, one technique alternately appliescircumferential and longitudinal magnetizing fields to the cylindricalarticle while plating isotropic film layers. The longitudinal fieldcomponent is derived from a Helmholtz coil pair. A circumferential fieldis developed at discrete intervals by a current which is caused to panthrough the cylindrical substrate preferably with the hollow form shownin FIG. 11. Substantial magnetic isotropy can be obtained in a NiFemagnetic layer where said external magnetic fields are altered at a rateof once for each A. or less throughout the deposition of the isotropicmagnetic layer when using electroless deposition previously described.Likewise, uniaxiality is obtained by applying continuous external fieldsof fixed direction during deposition of the anisotropic magnetic layers.

Various methods may be devised to establish antiparallel magnetizationvectors in the various magnetic storage devices. The preferred form,particularly in the embodiment of FIG. 5 where both magnetic layers haveeasy axes parallel with the substrate longitudinal axis, a word pulse isapplied to and maintained on the word line; i.e., the conductivesubstrate (see FIG. 7) for a time duration great enough to be concurrentwith a bit pulse sequence on the bit-sense line (FIG. 7), where a bitpulse is first applied in one direction and then reversed. Thiscombination of pulses produces switching in the upper anisotropic layerof FIG. 5 while the magnetization vector of the bottom film is forced tothe antiparallel state by the demagnetizing fields produced by the topfilm whose rest direction is defined by the polarity of the terminal bitpulse.

In the deposition of the copper barrier layers, a technique ofelectroplating is preferred in which fine grain' copper is depositedelectrolytically on the various magnetic layers. One such process fordepositing the copper layers comprises applying a plating currentdensity to the substrate of the memory element of 16 ma./in. in anelectrolytic copper bath formed from a mixture having a ratio of l07-644g. of Cu P O solution to which is added from 9-56 g. of NaCl. One suchcopper bath solution to which NaCl is added is commercially availableunder the brand name of unichrome. The bath is maintained at a pH levelslightly above 7; e.g., 7.75, and operated within a temperature range of2050 C.

.In summary, the elimination of both hard and easy axis demagnetizingfield components H in the improved memory element of this inventionproduces the following advantages over the prior art: (I) Use of twothicker films which cooperate to produce larger sense signals because ofthe greater number of lines of flux which can be switched in response tothe applied word field H (2) Reduction of the value of the magnetizingfield H and consequently the energizing current I required to switch theelement; (3) Cancellation of orthogonal demagnetizing field componentswhich otherwise would cause irreversible switching in portions of thefilm upon rotation of the magnetization vector toward the hard axis,which otherwise renders the device impractical for NDRO application; (4)Magnetostatic stabilization of the film in the unenergized statesleading to a much greater percentage of the film capable of respondingcoherently to energization which could not occur when the demagnetizingfield approaches the coercivity H of the film throughout its crosssection; (5) Provide an NDRO memory element capable of tolerating wordcurrent pulses which can exceed the intrinsic anisotrophy field of thestorage film without the usual deleterious effect of losing theinformation states which may occur spuriously within a memory system,thus allowing the memory devices of this invention to be utilized in areliable electrically alterable read-only memory configuration; (6)Provide an l3 NDRO memory element 55% of stable operation whileexperiencing stray fields of the order of l oersted or so, even if 'saidstray fields are oriented parallel with the easy magnetic axis. Thistolerance to stray fields eliminates the necessity to shield an array ofmemory elements from earth or other fields.

While the invention has been particularly shown I and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

I claim:

1. A bistable magnetic data storage device comprising:

a cylindrical conductive substrate of nonmagnetic material;

a data storage medium on said substrate comprising at least a first,second, and third closed, concentric, and longitudinally coextensivelayers;

said first and third layers comprising an alloy including NiFe which ismagnetic;

said first and third layers having a coercivity to anisotropy fieldratio within the range of 0.5 and 1.3 and a skew angle not to exceed 3;

said first and third layers being anisotropic with an easy axisorientation parallel with said axis of said substrate;

said first and third layers having magnetization vectors directed inantiparallel mode; and

said second layer which is between said first and third layerscomprising nonmagnetic conductive materials, said second layer having athickness thin enough to pennit mutual coupling of the easy axisdemagnetizing components of said magnetic layers and thick enough toeliminate magnetic exchange coupling between said magnetic layers andcapable of sustaining circumferential eddy currents whereby a solenoidalwhereby is generatedparallel with the axis of said substrate uponrotation of the magnetization vectors of said first and third layers.

2. A bistable magnetic data storage device comprising:

- a cylindrical substrate of conductive nonmagnetic material;

a data storage medium superimposed on said substrate comprising at leasta first, second, and third circumferentially continuous, concentric andlongitudinally coextensive layers;

said first and third layers comprising an alloy including NiFe;

said first and third layers each having an easy axis orientationsubstantially parallel with said longitudinal substrate;

and

said second layer which is between said first and third layerscomprising a material which-is nonmagnetic and which forms a barrierlayer between said first and third layers;

said second layer being thin enough to permit mutual coupling of theeasy axis demagnetizing components of said magnetic layer and thickenough to substantially eliminate transverse magnetic exchange couplingbetween said magnetic layers.

3. A magnetic storage device in accordance with claim 2 in which saidsecond layer has a thickness greater than 200 A.

4. A bistable magnetic data storage device in accordance with claim 2 inwhich said first and third magnetic layers have substantially equalanisotropy field strength; and

said second layer is a conductive material having a thickness furthersufficient to sustain circumferential eddy currents whereby a solenoidalfield is generated parallel with the axis of said substrate uponrotation of the magnetization vectors of said first and third magneticlayers.

5. A bistable magnetic data storage device in accordance with claim 2 inwhich said first and third magnetic layers have unequal anisotropy fieldstrength.

6. A bistable magnetic data storage device in accordance with claim 5 inwhich said second layer is a conductive material.

7. A bistable magnetic storage device in accordance with claim 5 inwhich said second layer is a dielectric material.

8. A bistable magnetic data storage device comprising in combination:

a cylindrical substrate of conductive nonmagnetic material;

a data storage medium on said substrate comprising first,

second, and third closed concentric magnetic layers;

said magnetic layers comprising an alloy including Nil-e;

each of said magnetic layers having a preferred direction of magneticorientation parallel with the axis of said substrate; and

said storage medium further comprising conductive barrier layersseparating said magnetic layers;

said conductive barrier layers being thin enough to permit mutualcoupling of the easy axis demagnetizing components "of said magneticlayers and thick enough to substantially eliminate transverse magneticexchange coupling between said magnetic layers;

said magnetic and said barrier layers being longitudinally coextensive.

9. A bistable magnetic data storage device in accordance with claim 8 inwhich said third layer is the outer layer and has a magnetic thicknesssubstantially equal to the combined magnetic thickness of said first andsecond layers.

10. A bistable magnetic data storage element in accordance with claim 9in which said first and second inner magnetic layers have magnetizationvectors oriented parallel with each other and said third magnetic layerhas a magnetization vector antiparallel with said magnetization vectorsof said first and second layers.

22 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3576 552 Dated April 27 1971 Inventor(s) Albert W. Vinal It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

F- Column 6, line 48, the formula reading M xAD =M xD eq. (2)

should read M xD =M xD eq. (2)

Column 10, line 8 the formula reading s a s i cos (3) should read s a s.i cos 6i (3) Claim 1, column 13, line 28, delete "having a thickness"line 33, delete "whereby" after "solenoidal" and insert field.

Signed and sealed this 5th day of December 1972.

(SEAL) .Attest:

EDWARD M.FLETCI-IER,JR. ROBERT GOTTSGHALK Attesting Officer Commissionerof Patent

1. A bistable magnetic data storage device comprisiNg: a cylindricalconductive substrate of nonmagnetic material; a data storage medium onsaid substrate comprising at least a first, second, and third closed,concentric, and longitudinally coextensive layers; said first and thirdlayers comprising an alloy including NiFe which is magnetic; said firstand third layers having a coercivity to anisotropy field ratio withinthe range of 0.5 and 1.3 and a skew angle not to exceed 3*; said firstand third layers being anisotropic with an easy axis orientationparallel with said axis of said substrate; said first and third layershaving magnetization vectors directed in antiparallel mode; and saidsecond layer which is between said first and third layers comprisingnonmagnetic conductive materials, said second layer having a thicknessthin enough to permit mutual coupling of the easy axis demagnetizingcomponents of said magnetic layers and thick enough to eliminatemagnetic exchange coupling between said magnetic layers and capable ofsustaining circumferential eddy currents whereby a solenoidal whereby isgenerated parallel with the axis of said substrate upon rotation of themagnetization vectors of said first and third layers.
 2. A bistablemagnetic data storage device comprising: a cylindrical substrate ofconductive nonmagnetic material; a data storage medium superimposed onsaid substrate comprising at least a first, second, and thirdcircumferentially continuous, concentric and longitudinally coextensivelayers; said first and third layers comprising an alloy including NiFe;said first and third layers each having an easy axis orientationsubstantially parallel with said longitudinal substrate; and said secondlayer which is between said first and third layers comprising a materialwhich is nonmagnetic and which forms a barrier layer between said firstand third layers; said second layer being thin enough to permit mutualcoupling of the easy axis demagnetizing components of said magneticlayer and thick enough to substantially eliminate transverse magneticexchange coupling between said magnetic layers.
 3. A magnetic storagedevice in accordance with claim 2 in which said second layer has athickness greater than 200 A.
 4. A bistable magnetic data storage devicein accordance with claim 2 in which said first and third magnetic layershave substantially equal anisotropy field strength; and said secondlayer is a conductive material having a thickness further sufficient tosustain circumferential eddy currents whereby a solenoidal field isgenerated parallel with the axis of said substrate upon rotation of themagnetization vectors of said first and third magnetic layers.
 5. Abistable magnetic data storage device in accordance with claim 2 inwhich said first and third magnetic layers have unequal anisotropy fieldstrength.
 6. A bistable magnetic data storage device in accordance withclaim 5 in which said second layer is a conductive material.
 7. Abistable magnetic storage device in accordance with claim 5 in whichsaid second layer is a dielectric material.
 8. A bistable magnetic datastorage device comprising in combination: a cylindrical substrate ofconductive nonmagnetic material; a data storage medium on said substratecomprising first, second, and third closed concentric magnetic layers;said magnetic layers comprising an alloy including NiFe; each of saidmagnetic layers having a preferred direction of magnetic orientationparallel with the axis of said substrate; and said storage mediumfurther comprising conductive barrier layers separating said magneticlayers; said conductive barrier layers being thin enough to permitmutual coupling of the easy axis demagnetizing components of saidmagnetic layers and thick enough to substantially eliminate transversemagnetic exchange coupling between said magnetic layers; said magneticand said barrier layers beIng longitudinally coextensive.
 9. A bistablemagnetic data storage device in accordance with claim 8 in which saidthird layer is the outer layer and has a magnetic thicknesssubstantially equal to the combined magnetic thickness of said first andsecond layers.
 10. A bistable magnetic data storage element inaccordance with claim 9 in which said first and second inner magneticlayers have magnetization vectors oriented parallel with each other andsaid third magnetic layer has a magnetization vector antiparallel withsaid magnetization vectors of said first and second layers.